1. Field of the Invention
The present invention relates to a lens for projection and a projection-type display apparatus, and particularly to a lens for projection substantially consisting of seven lenses and a projection-type display apparatus using the lens for projection.
2. Description of the Related Art
In recent years, projection-type display apparatuses (also called as projectors) for projecting, onto screens, images represented by image data output from personal computers or the like rapidly spread. As such projectors, a projector that projects, onto a screen, light that has been output from a light source and modulated by a light valve is known. As the light valve, a transmission-type liquid crystal device, a reflection-type liquid crystal device, a DMD (Digital Micromirror Device), and the like are known. In the DMD, micromirrors, the angles of which are changeable, are arranged.
Such a projector needs to have a space for arranging an optical path for guiding light output from the light source to the light valve and an optical path for guiding light modulated by the light valve to the lens for projection. Therefore, a lens for projection mounted on the projector has a long back focus so that such a space is provided between the lens for projection and the light valve.
For example, a projector for projecting color images, and which includes light valves for modulating red light, green light and blue light respectively, needs to have a space for arranging optical paths for guiding light of each color to respective light valves and optical paths for guiding light modulated by the light valves to a lens for projection.
As a lens for projection in which a back focus is set to provide such a space, a lens for projection composed of seven lenses, and which has an F-number in the range of 2.4 to 6.4 is known (please refer to Japanese Unexamined Patent Publication No. 2003-287676 (Patent Document 1), Japanese Unexamined Patent Publication No. 2005-164839 (Patent Document 2), Japanese Unexamined Patent Publication No. 2005-173494 (Patent Document 3), U.S. Pat. No. 5,745,297 (Patent document 4), Japanese Patent No. 4076334 (Patent Document 5), and Specification of U.S. Pat. No. 5,731,676 (Patent document 6)).
Further, although a back focus is short, a lens for projection composed of seven lenses with a small F-number of 1.7 (fast lens) is known (please refer to Specification of U.S. Pat. No. 7,057,830 (Patent Document 7)).
Here, when the number of lenses is counted, if a cemented lens composed of n lenses is included, the number of lenses of the cemented lens is regarded as n.
When images are projected onto a screen through a projector, a setting place of the screen is generally made dark, but a demand for watching images without making the place very dark is strong. However, projectors in which lenses for projection having relatively large F-numbers, as disclosed in Patent Documents 1 through 6, are mounted do not meet such a demand. Meanwhile, a projector in which a lens for projection having a small F-number (fast lens), as disclosed in Patent Document 7, is mounted may meet the demand, but there is a problem that the back focus of the lens for projection is short.
Specifically, in the lenses for projection disclosed in Patent Documents 1 through 6, if F-numbers are reduced (faster lenses) without changing the distribution of power balance of lenses (arrangement of positive and negative lenses), the size of apparatuses becomes large. Further, it becomes difficult to suppress generation of various aberrations, which lower the resolution of the lenses for projection.
In the lens for projection (F-number: 1.7) disclosed in Patent Document 7, if the back focus is increased without changing the distribution of power balance of lenses (arrangement of positive and negative lenses), the size of an apparatus becomes large. Further, it becomes difficult to suppress generation of various aberrations, which lower the resolution of the lens for projection.